1. Field of the Invention
This invention relates to a process of controlling the quality of sintered iron ore which has been produced in belt-type sintering plants provided with recycled-fines balance controllers which match the fines withdrawal rate with the recycled-fines feed rate.
2. Prior Art
The uniform operation of a blast furnace depends on a high and constant quality of the sintered iron ore.
The most important requirements to be met by the sintered iron ore are:
1. The chemical analysis of the sintered iron ore should be such that the blast furnace slag has a predetermined basicity;
2. The physical properties of the sintered iron ore should be such as to ensure a satisfactory behavior of the sintered iron ore in transit and in the blast furnace;
3. The reducibility of the sintered iron ore should by such that coke is consumed in the blast furnace at a low rate.
The chemical analysis of the sintered iron ore is selected by a proper mixing of ores and, if desired, admixtures. Large mixing beds are often used for this purpose and will ensure a homogenization to a high degree if they are carefully composed.
The remaining requirements regarding the quality of the sintered iron ore can be met only by the selection of suitable operating conditions and by the maintenance of a uniform operation.
In numerous plants, a control of the recycled-fines balance has been adopted to promote the uniformity ("Stahl und Eisen" 84 (1964), pages 1304-13). The recycled-fines balance control provides for a fuel rate which ensures a good recycled-fines balance in conjunction with a predetermined recycled-fines feed rate. Fluctuations in the recycled-fines balance are detected as changes of the recycled-fines bin content. A proportional-plus-integral controller changes the fuel rate in response to a change of the recycled-fines bin content. Because sintering plants operate with large time constants owing to the long handling times, the control response to a change of the recycled-fines bin content is improved in that the recycled-fines feed rate is temporarily changed too. A uniform operation with a good recycled-fines balance in conjunction with a predetermined fines withdrawal rate can be maintained with this control system.
The proportion of fines in the burden of a blast furnace depends on the mechanical strength of the sintered iron ore. To ensure a satisfactory behavior of the sintered iron ore in transit, as well as a regular operation of the blast furnace and a high throughput thereof, the strength of the sintered iron ore must not fall below a lower limit. The strength of the sintered iron ore is determined by known tests (ISO drum test) from samples taken once an hour, on an average.
The Fe.sup.++ content is also highly significant for the operation of the blast furnace because high Fe.sup.++ values lower the reducibility of the sintered iron ore so that the coke consumption in the blast furnace is excessive. Excessively low Fe.sup.++ contents involve a high recycled-fines feed rate so that the output of the sintering plant is decreased. Besides, the strength of the sintered iron ore is lower if the Fe.sup.++ content is below a certain limit, e.g., 6% Fe.sup.++. In most cases, the Fe.sup.++ content is determined in the laboratory by X-ray fluorescence analysis of suitable pretreated samples. A measurement of the magnetic properties of the sintered iron ore has been suggested, (Metallurgical Reports CNRM, No. 16, 1968, pages 11-18) and enables a continuous measurement of the Fe.sup.++ content in the plant. In this connection, it has been found suitable to provide for an automatic recalibration based on laboratory analyses.
Statistical investigations have shown that in case of a given mineralogical composition of the mixing bed, the strength of the sintered iron ore depends in a high degree on the height of the bed during the sintering operation. To ensure a strength above a lower limit, the height of the bed must be maintained above a minimum for a given mixture. On the other hand, it is not desirable to provide for a much higher bed for safety reasons because this would result in a loss in output. The height of the bed is substantially determined by these considerations.